1. Field of the Invention
The present invention relates to a phase monitor for a short-circuited asynchronous three-phase motor.
2. Description of the Related Art
More particularly, the invention relates to such a phase monitor of the kind comprising sensing means, intended to be connected to a supply voltage line to the motor in order to sense the voltage conditions on said line, a control switch, controlled by said sensing means and intended to be connected in a control circuit for a supply voltage switch for the motor in order to close and open said control circuit in dependence upon the sensed voltage conditions on said line, said supply voltage switch being connected in the supply voltage line at a location between the connection point of said sensing means and the motor, and delay means for delaying the switching-on and switching-off operations of the control switch.
Such phase monitors are commonly utilized to prevent damages and injuries caused by phase-sequence faults or phase voltage drop-outs in the supply voltage to the motor. Normally, phase-sequence faults do not occur in permanent installations, since, in the case of such installations, once the motor has been installed and connected in a correct manner to a power distribution network, the phase sequence will normally not be altered. However, in the case of portable equipments, such as those connected to the mains via plug contacts, there is a high risk of phase-sequence faults. Phase voltage drop-outs, on the other hand, may occur to the same extent at permanent installations as at portable equipments.
The delay means contained in the phase monitor have for their purpose to prevent normal voltage variations on the mains from causing alternating switching-on and switching-off operations of the control switch and hence also of the supply voltage switch for the motor.
However, when using phase monitors of the above kind previously known, the delay of the switching-on operation has been found to cause a serious problem. Especially in the case of portable equipments, said delay may very well result in that, when, as a consequence of said delay, the motor does not start immediately upon a connection of the supply voltage line to the mains, an operator may shift the phase sequence of the voltage applied on the supply voltage line and then leave the motor connected to a supply voltage having an incorrect phase sequence. The invention has for its purpose to provide an improved phase monitor of the kind initially specified which avoids the risk of such serios mistakes made by an operator.